This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. The goal of this subproject (phase 4) that we shall pursue in conjunction with the cryogenic receiver development task is to develop a cryogen-free probe head that will ensure the longevity of the cryogenic microwave components by relieving them from stresses originating from thermal cycling, (35 cycles per year, typical of our flow based system, carry a great potential for part degradation, based on literature sources and our experience). The system will reduce costs of operation by eliminating cryogen and will provide a continuous mode of operation. In this phase, we shall also design and fabricate a cryogen-free ESR cryostat with the sample in helium gas. Our current plan is to utilize a single PT for the receiver and the probe-head compartments. The latter will use gaseous helium and a closed-loop hermetic scroll-compressor based helium circulation system (refrigerator) using cold production capacity of the two stages of the PT. The circulating helium gas and heat-exchange helium gas surrounding the samples are both supplied from a standard high-pressure cylinder. This is similar to sample-in-vapor accessory offered by Cryogenics, LTD.